Illumination systems are used as either stand-alone light sources or as internal light sources for more complex optical systems. Examples of optical systems that utilize or incorporate illumination systems include projection displays, flat-panel displays and avionics displays.
Many applications require illumination systems with high brightness and a small effective emitting area. An example of a conventional light source with high brightness and a small effective emitting area is an arc lamp source, such as a xenon arc lamp or a mercury arc lamp. Arc lamp sources may have emitting areas as small as a few square millimeters. An example of a complex optical system that can utilize an illumination system with high brightness and a small effective source area is a projection display system. Early projection display systems typically project the combined images of three small red, green and blue cathode-ray-tube (CRT) devices onto a viewing screen using projection lenses. More recent designs sometimes use a small-area arc lamp source to project images from a liquid crystal display (LCD) device, a liquid-crystal-on-silicon (LCOS) device or a digital light processor (DLP) device onto a viewing screen. DLP devices utilize an array of micro-mirrors to form an image.
An arc lamp source has a fixed color temperature and a brightness that is difficult to adjust. LED light sources are more flexible. An LED light source that includes mixtures of red, green and blue LEDs can be adjusted to change the color temperature and overall brightness of the source. However, LEDs are currently not used for large projection display systems because LED sources do not have sufficient output brightness.
The technical term brightness can be defined either in radiometric units or photometric units. In the radiometric system of units, the unit of light flux or radiant flux is expressed in watts and the unit for brightness is called radiance, which is defined as watts per square meter per steradian (where steradian is the unit of solid angle). The human eye, however, is more sensitive to some wavelengths of light (for example, green light) than it is to other wavelengths (for example, blue or red light). The photometric system is designed to take the human eye response into account and therefore brightness in the photometric system is brightness as observed by the human eye. In the photometric system, the unit of light flux as perceived by the human eye is called luminous flux and is expressed in units of lumens. The unit for brightness is called luminance, which is defined as lumens per square meter per steradian. The human eye is only sensitive to light in the wavelength range from approximately 400 nanometers to approximately 700 nanometers. Light having wavelengths less than about 400 nanometers or greater than about 700 nanometers has zero luminance, irrespective of the radiance values.
In U.S. Pat. No. 6,869,206, brightness enhancement referred to luminance enhancement only. Since luminance is non-zero only for the visible wavelength range of 400 to 700 nanometers, U.S. Pat. No. 6,869,206 is operative only in the 400- to 700-nanometer wavelength range visible to the human eye. In U.S. patent application Ser. No. 10/814,043 entitled “ILLUMINATION SYSTEMS UTILIZING LIGHT EMITTING DIODES AND LIGHT RECYCLING TO ENHANCE OUTPUT RADIANCE,” brightness enhancement refers to radiance enhancement and is valid for any wavelength throughout the optical spectrum. In this application, brightness enhancement will generally refer to luminance enhancement.
In a conventional optical system that transports light from an input source at one location to an output image at a second location, one cannot produce an optical output image whose luminance is higher than the luminance of the light source. A conventional optical system 20 of the prior art is illustrated in cross-section in FIG. 1. In FIG. 1, the input source 22 has area, Areain. The light rays from input source 22 fill a truncated cone having edges 21 and 23. The cone, which is shown in cross-section in FIG. 1, extends over solid angle 27. The magnitude of solid angle 27 is Ωin. Lens 24 focuses the light rays to image 26 having area, Areaout. The light rays forming the image 26 fill a truncated cone having edges 25 and 29. The cone, which is shown in cross-section, extends over solid angle 28. The magnitude of solid angle 28 is Ωout.
If the optical system 20 has no losses, the light input flux at the input source 22,Φin=(Luminancein)(Areain)(Ωin),   [Equation 1]equals the light output flux at the output image 26,Φout=(Luminanceout)(Areout)(Ωout).   [Equation 2]In these equations, “Luminancein” is the luminance at the input source 22, “Luminanceout” is the luminance at the output image 26, “Areain” is the area of the input source 22 and “Areaout” is the area of the output image 26. The quantities Ωin and Ωout are, respectively, the projected solid angles subtended by the input source and output image light cones. In such a lossless system, it can be shown thatLuminancein=Luminanceout   [Equation 3]and(Areain)(Ωin)=(Areaout)(Ωout)   [Equation 4]If the index of refraction of the optical transmission medium is different at the input source and output image positions, the equality in Equation 4 is modified to become(nin2)(Areain)(Ωin)=(nout2)(Areaout)(Ωout)   [Equation 5]where nin is the index of refraction at the input position and nout is the index of refraction at the output position. The quantity (n2)(Area)(Ω) is variously called the “etendue” or “optical extent” or “throughput” of the optical system. In a conventional lossless optical system, the quantity (n2)(Area)(Ω) is conserved and Luminancein equals Luminanceout . However, under certain conditions utilizing light recycling, the effective luminance of the source as well as the maximum exiting luminance of the optical system can be higher than the intrinsic luminance of the source in the absence of recycling, a result that is not predicted by the standard etendue equations.
Recently, highly reflective green, cyan, blue and ultraviolet LEDs and diode lasers based on gallium nitride (GaN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN) and aluminum nitride (AlN) semiconductor materials have been developed. Some of these LED devices have high light output, high luminance and have a reflecting layer that can reflect at least 50% of the light incident upon the device. Such a reflecting layer is necessary in order to increase the effective luminance of the LED by light recycling. The reflecting layer of the LED can be a specular reflector or a diffuse reflector. Typically, the reflecting layer of the LED is a specular reflector. Luminance outputs of several million lumens per square meter per steradian and total outputs greater than 100 lumens from a single packaged device are possible. Light outputs per unit area can exceed 25 lumens per square millimeter. As such, several new applications relating to illumination systems have become possible. Advantages such as spectral purity, reduced heat, and fast switching speed all provide motivation to use LEDs and semiconductor lasers to replace fluorescent, incandescent and arc lamp sources.
Red and yellow LEDs were developed earlier than the UV, blue, cyan and green LEDs. The red and yellow LEDs are generally made from a different set of semiconductor materials, one particular example being aluminum indium gallium phosphide (AlInGaP).
FIG. 2 illustrates a cross-sectional view of a recently developed type of LED 40 that has an emitting layer 46 located below both a partially transparent conducting layer 43 and a transparent growth substrate layer 44. The growth substrate 44 is the original substrate onto which the semiconducting layers are grown by epitaxial deposition means. Emitting layer 46 emits light rays 45 when an electric current is passed through the device 40. Below the emitting layer 46 are a second partially transparent conducting layer 49 and a reflecting layer 47 that also serves as a portion of the bottom electrode. Electrical contacts 41 and 42 provide a pathway for electrical current to flow through the device 40. The reflecting layer 47 allows the LED to be both a light emitter and a light reflector. Lumileds Lighting LLC, for example, produces highly reflective green, blue and ultraviolet LED devices of this type. It is expected that highly reflective yellow, red and infrared LEDs with high outputs and high luminance will also eventually be developed. However, even the new green, cyan, blue and ultraviolet gallium nitride, indium gallium nitride, aluminum gallium nitride and aluminum nitride LEDs do not have sufficient luminance for many applications.
LEDs, including inorganic light-emitting diodes and organic light-emitting diodes, emit incoherent light. On the other hand, semiconductor laser light sources, such as edge-emitting laser diodes and vertical cavity surface emitting lasers, generally emit coherent light. Coherent semiconductor laser light sources typically have higher brightness than incoherent light sources, but semiconductor laser light sources are not suitable for many applications such as displays due to the formation of undesirable speckle light patterns that result from the coherent nature of the light.
Most light-emitting color projection displays utilize three primary colors to form full-color images. The three primary colors are normally red (R), green (G) and blue (B), but some projection displays may also utilize additional colors such as white (W), yellow (Y), cyan (C) and magenta (M). The red, green and blue primary colors can be mixed to form thousands or millions of colors. However, such systems do not reproduce all the colors that a human eye can visualize. The colors that can be visualized by the human eye can be graphed in X and Y color coordinates as the 1931 CIE Chromaticity Diagram. A representation of the 1931 CIE Chromaticity Diagram is shown in FIG. 3A. The X and Y color coordinates of the pure colors, such as 700 nm, 600 nm, 500 nm and 400 nm are points on the “curved line of pure colors” in FIG. 3A. The straight line connecting the 400-nm and 700-nm points is the “line of purples”, which are mixtures of 400-nm and 700-nm light. The enclosed area inside the “curved line of pure colors” and “line of purples” represents all the colors that are visible to the human eye. All the colors inside the enclosed area that are not on the curved line are mixtures of pure colors.
A cathode ray tube (CRT) computer monitor utilizes red, green and blue phosphors to display multicolor images. The approximate color coordinates for the resulting R, G and B primary colors are shown in FIG. 3A and form a triangle. Notice that there is considerable area outside the RGB triangle that falls within the range of colors visible to the human eye and represents colors that cannot be reproduced by the computer monitor. The shaded area inside the triangle represents all the colors that can be formed by mixing varying amounts of the R, G, and B primary colors. This shaded are is called the color gamut for a CRT computer monitor.
The total number of mixed colors and color grayscale levels that can be produced by a CRT monitor depends on the number of intensity levels that can be produced for each R, G and B color. For example, the line between R and G represents colors that can be produced by mixing only R and G. If the monitor can produce, for example, 100 intensity levels (grayscale levels) of R and 100 intensity levels (grayscale levels) of G, then R and G can be mixed 100×100 or 10,000 ways to produce many different colors and many different grayscale levels of particular colors. When R and G are mixed, the resulting color depends on the ratio of R to G. The grayscale level of the mixed color depends on the intensity level of the mixture. As an illustrative example, mixing intensity level 100 of the color R and intensity level 100 of the color G can produce the color yellow. The ratio of intensity level R to intensity level G is 100:100 or 1:1. Mixing intensity level 50 of the color R and intensity level 50 of the color G will produce the same yellow color since the ratio of the two intensity levels is still 1:1. However, the intensity or grayscale level of the 50:50 mixture is one-half of the intensity or grayscale level of the 100:100 mixture. Adding a third primary color B increases the number of possible colors. In this example, if the total number of intensity or grayscale levels of B is 100, then R, G and B can be mixed 100×100×100 or 1,000,000 ways to achieve a wide range of colors and multiple grayscale levels of the same color. The colors that are called white are mixtures of R, G and B and are located in the central region of the RGB triangle.
One can increase the color gamut of a display system by adding additional colors located outside the RGB triangle. For example, if one adds yellow (Y) and cyan (C) colors that have color coordinates outside the RGB triangle, the shaded area corresponding to the color gamut increases as shown in FIG. 3B. Therefore a wider range of colors can be produced by a display system that uses five primary colors (R, G, B, Y and C) than by a display system that uses three primary colors (R, G and B).
It would be highly desirable to develop LED-based projection display systems that utilize light recycling in order to increase the maximum output luminance of the systems. It would also be desirable to use LEDs to extend the color gamut and grayscale range of projection display systems. Furthermore, it would be desirable to develop LED-based displays where the color temperature and overall brightness of the display can be modified as desired. Possible uses include front and rear projection displays for television and avionics applications.